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Simple security by evaluating open ports on Linux

"A simple but effective procedure for evaluating security on
your computer is to check what sites it's connecting to, or what
sites are connecting to it. Most critical malware nowadays turn
computers into zombies for botnets — typically zombified
hosts will connect to a central server using IRC. Or it could be
that you're inadvertently running a program that's listening for
Internet requests. In any case, it's good to check these
connections."